1. Field of the Invention
The invention relates to a dual-mode receiver, and more particularly to a receiver for a wideband system and a narrowband system, which combines a direct-conversion mode and a low-IF mode and can alternate therebetween.
2. Description of the Related Art
Multi-mode receivers have been proposed during the past few years. Multi-mode receivers can handle two or more communication standards, such as a mobile phone supporting a 3G standard (such as WCDMA) and a 2G standard (such as GSM). Because of various specifications of communication systems, channel bandwidths are different, such as the bandwidths of the WCDMA system and the GSM system being respectively 5 MHz and 200 kHz.
Two structures for highly integrated receivers are low-IF receiver and direct-conversion receiver. The low-IF receiver can avoid the problems of DC offset and low frequency noise, but may interfere with image signals. On the contrary, the direct-conversion receiver can avoid image signals, but may be influenced by the DC offset and low frequency noise. Narrowband signals received by direct-conversion receivers are usually influenced by the DC offset and low frequency noise. Therefore, it is easier to receive narrowband signals with high quality by low-IF receivers. Wideband signals received by direct-conversion receivers are rarely influenced by the DC offset and low frequency noise. Therefore, using direct-conversion receivers to receive the wideband signals can decrease power consumption and the bandwidth for processing analog baseband signals.
At present, RF transceivers often use BiCMOS elements. CMOS elements are another choice for making RF transceivers. An advantage is that CMOS RF transceivers are easy to integrate with CMOS digital circuits to realize system-on-chip (SOC). Flicker noise on the CMOS element is more, however, than on the BJT element, such that it is much more difficult to make a direct-conversion receiver for a narrow system with COMS elements.
FIG. 1 is a diagram illustrating the conventional configuration of the direct-conversion receiver. A signal SI is received by an antenna 102. The frequency of the carrier of the signal SI is fc. After passing a bandpass filter 104, the signal SI is amplified by a low noise amplifier 106. The quadrature mixer 108 converts the amplified signal down to a pair of baseband signals. The baseband signals are respectively an I-channel signal SID and a Q-channel signal SQD. A mixer 128a of the quadrature mixer 108 receives a sine signal whose frequency is fc and whose phase is 0° from a local oscillator 109. A mixer 128b of the quadrature mixer 108 receives a sine signal whose frequency is fc and whose phase is 90° from the local oscillator 109.
After passing DC offset cancellation units 110a and 110b, the I-channel signal SID and the Q-channel signal SQD are respectively input to lowpass filter 112a and 112b. Then, the filter signals are respectively input to programmable gain amplifiers 114a and 114b to generate an I-channel signal SIO and a Q-channel signal SQO output. The output signals are usually input to a digital signal processor (not shown in FIG. 1). The direct-conversion receiver can avoid image signals, but may be influenced by the DC offset and low frequency noise. If the direct-conversion receiver is used in a narrowband system such as a GSM system, the DC offset will overlap the downconverted signal and the signal-to-noise ratio will be decreased. Although a GSM product using a direct-conversion receiver is available (referring to HD155141TF, Hitachi), the product requires extra current for complex analog signal processing technology to cancel DC offset. On the contrary, wideband signals such as WCDMA signals are rarely influenced by the DC offset and low frequency noise. The bandwidth of the analog baseband circuits in the low-IF receiver is two or more times that of the direct-conversion receiver. Therefore, using direct-conversion receivers to receive the wideband signals can decrease power consumption and required bandwidth for processing analog baseband signals.
FIG. 2 is a diagram illustrating the conventional configuration of the low-IF receiver. The elements of the low-IF receiver and of the direct-conversion receiver with the same function have the same labels. The major difference between the low-IF receiver and the direct-conversion conversion receiver is that the quadrature mixer 208 converts the amplified signal down to a pair of intermediate frequency signals with the carrier whose frequency is fIF (fIF is usually half the channel bandwidth but is not limited thereto). The secondary downconverter converts the intermediate frequency signals to a pair of baseband signals SIO and SQO output. A mixer 228a of the quadrature mixer 208 receives a sine signal whose frequency is fc-fIF and whose phase is 0° from a local oscillator 209. A mixer 228b of the quadrature mixer 208 receives a sine signal whose frequency is fc-fIF and whose phase is 90° from the local oscillator 209. Furthermore, image rejection units 211a and 211b are added in the low-IF receiver to cancel image rejection. Although DC offset in the low-IF receiver must be canceled, it is not as important as in the direct-conversion receiver. The bottleneck of the low-IF receiver is that the canceled amount of the image rejection depends on the matched level of the elements in the low-IF receiver chip. At present, the maximum amount is around 30 dB. Therefore, for systems whose adjacent channel has high power such as WCDMA system, the difficulty for using low-IF receivers is significantly increased.
Accordingly, the low-IF receiver and the direct-conversion receiver each have their own advantages. Using the low-IF receiver to receive narrowband signal can avoid the problems of DC offset and low frequency noise. Using the direct-conversion receiver to receive the wideband signals can decrease power consumption and the bandwidth for processing analog baseband signals.